Acne develops when your oil glands produce too much sebum, dead skin cells accumulate inside pores, and bacteria trigger inflammation. Those four factors work together, but what sets them in motion varies from person to person. Roughly 85% of people between ages 12 and 25 experience some form of acne, and it continues well into adulthood for many, affecting up to 20% of women and 8% of men past their mid-twenties.
What Happens Inside a Pore
Every pore on your face and body contains a hair follicle and a tiny oil gland. These glands produce sebum, a waxy substance that keeps your skin lubricated. In a healthy pore, sebum flows up and out onto the skin’s surface without any trouble.
Acne starts when two things go wrong at once. First, the oil glands ramp up production and make more sebum than the pore can handle. Second, the skin cells lining the inside of the pore shed faster than normal and stick together instead of sloughing off. This combination creates a plug. Behind that plug, sebum keeps building up, forming a whitehead (closed) or blackhead (open to the air). If bacteria that naturally live on your skin get trapped behind the plug and multiply, your immune system responds with inflammation, and the clogged pore becomes a red, swollen pimple. When that inflammation reaches deeper layers of skin, it can form painful cysts or nodules.
Hormones Are the Biggest Driver
Androgens, a group of hormones that includes testosterone, are the primary reason oil glands overproduce. During puberty, androgen levels surge in both boys and girls, which is why acne peaks in the teenage years and is slightly more common in adolescent males. Androgens bind to receptors on oil gland cells and trigger them to grow larger, produce more fat-like molecules, and push out more sebum. The most potent androgen in this process is dihydrotestosterone (DHT), which directly stimulates oil gland cells to synthesize and store lipids in large droplets.
This is also why acne flares around menstrual cycles, during pregnancy, and with conditions like polycystic ovary syndrome. Any shift that increases androgen activity or decreases the proteins that keep androgens in check can tip the balance toward oilier skin and more breakouts.
Genetics Set Your Baseline Risk
If one or both of your parents dealt with acne, your chances are significantly higher. A study in the British Journal of Dermatology found that having a first-degree relative with adult acne nearly quadrupled the odds of developing it yourself (odds ratio of 3.93). Genetics influence how large your oil glands are, how sensitive they are to hormones, how quickly your skin cells turn over, and how aggressively your immune system reacts to clogged pores. You can’t change your genetic blueprint, but understanding it helps explain why some people break out from triggers that leave others unaffected.
How Diet Feeds Into Breakouts
The link between diet and acne centers on insulin and a related hormone called insulin-like growth factor 1 (IGF-1). When you eat foods that spike your blood sugar quickly, like white bread, sugary drinks, or processed snacks, your pancreas floods your bloodstream with insulin. Chronically elevated insulin raises free IGF-1 levels, and IGF-1 does two things that matter for acne: it stimulates oil glands to produce more sebum, and it amplifies the effect of androgens on those same glands. Research has confirmed a positive correlation between IGF-1 levels in the blood and the rate of sebum production on the face.
Dairy is a separate but related trigger. Milk naturally contains IGF-1 along with other hormones like prolactin and steroids. The two main proteins in milk, whey and casein, each contribute differently. Whey raises blood insulin levels, while casein boosts IGF-1. In the United States, many dairy cows are also treated with synthetic growth hormones that further increase IGF-1 concentrations in milk. This doesn’t mean dairy causes acne in everyone, but for people who are already acne-prone, it can make things worse.
Stress Makes Existing Acne Worse
Psychological stress doesn’t cause acne on its own, but it reliably worsens it. When you’re stressed, your body produces corticotropin-releasing hormone (CRH) and cortisol. Oil glands have receptors for both. Research has found very strong expression of CRH in the oil glands of acne-affected skin compared to clear skin nearby. CRH directly stimulates sebum production and also activates an enzyme that converts weaker hormones into active androgens right at the gland itself. So stress essentially creates a local hormone surge in the skin, even if your overall hormone levels in the blood haven’t changed much.
Bacteria: Not All Strains Are Equal
The bacterium most associated with acne lives on virtually everyone’s skin. It feeds on sebum and thrives in the low-oxygen environment of a plugged pore. But having this bacterium on your skin doesn’t automatically mean you’ll break out. Recent genetic research has revealed that certain strains carry a small piece of extra DNA, a linear plasmid, that makes them far more inflammatory than other strains of the same species. When these high-inflammation strains colonize a clogged pore, they provoke a much stronger immune response in surrounding skin cells, which is what turns a simple clogged pore into an angry, red, painful lesion. This helps explain why two people with equally oily skin can have very different acne severity.
Your Skin Barrier Plays a Role Too
People with acne tend to have a compromised skin barrier. Studies measuring transepidermal water loss (essentially how much moisture escapes through the skin) have found that acne patients lose water through their skin at higher rates than the general population, alongside elevated sebum levels. A weakened barrier shifts the skin’s surface environment in ways that can encourage bacterial overgrowth and make inflammation harder to resolve. This is one reason harsh cleansers and over-scrubbing often backfire. Stripping the skin damages the barrier further, which can trigger even more oil production as the skin tries to compensate.
Why Acne Persists Into Adulthood
Acne isn’t strictly a teenage problem. The global prevalence across all age groups sits around 9.4%, and adult acne is increasingly common. In women, hormonal fluctuations tied to menstrual cycles, oral contraceptive changes, perimenopause, and conditions that elevate androgens keep oil glands active well past adolescence. Chronic stress, dietary habits, and ongoing genetic predisposition all contribute. Adult acne tends to appear along the jawline and chin rather than the forehead and nose pattern typical of teenagers, partly because the oil glands in those areas are more hormone-sensitive.
Putting the Causes Together
Acne is rarely caused by a single factor. For most people, it’s a combination: your genetics determine how reactive your oil glands and immune system are, hormones set the level of oil production, diet and stress can amplify that production, and the specific bacteria on your skin determine how much inflammation follows. This layered nature is why what works for one person’s acne may not work for another, and why effective management often involves addressing more than one trigger at a time.